1. Field of the Invention
The invention relates to a heat exchanger in a coolant circuit of a motor vehicle and system to heat the passenger compartment.
2. Related Art
The trend towards highly efficient motor vehicle drive systems has resulted in a lack of sufficient waste heat for heating the interior of the vehicle. Therefore, the comfort conditions get worse in vehicles in which the passenger compartment is heated solely based on the engine's coolant circuit.
In the state-of-the-art there are many approaches to solve this problem. For example, the cooling circuit can be electrically heated or the air of the vehicle interior can be directly heated by means of PTC resistors. Additionally, fuel-fired supplemental heating devices for the coolant circuit are known.
An alternative development to supplemental heating devices is to use the refrigerant systems, or air conditioning units, present in motor vehicles for the heating of the vehicle interior. This is possible by operating the air conditioning unit as heat pump. Alternatively, a “short” circuit, without secondary heat absorption in the clockwise-rotating version or anticlockwise-rotating version, can be used. In a short circuit, essential portions of the mechanical drive power of the compressor are transformed into heat for the purpose of heating the passenger compartment. Such air conditioning units are also known.
When an air conditioning unit in a vehicle is used for additional heating, a highly undesirable effect occurs under certain use and environmental conditions. Particularly, when the refrigerant system is used as a cooling plant, the evaporator arranged in the ventilating system of the vehicle will dehumidify the air to be cooled. After having stopped the engine and starting it anew when the heat exchanger has previously been used as evaporator and now is subsequently used as condenser or gas cooler in heating modes, due to heat being given off to the air flow, the humidity condensed on the evaporator surface will be introduced into the vehicle interior. Alternating use of the system as cooling plant and heat pump is quite frequent in the transitional weather periods, such as Spring and Autumn.
The high humidity air led into the vehicle interior results in condensation on the cold interior surfaces of the vehicle, particularly on the windows, with accompanying deterioration of the passengers' sight. This effect is also called flash-fogging.
In the state-of-the-art, solutions exist that are intended to prevent this effect.
After a special form of construction for the refrigerant carbon dioxide, in DE 198 55 309 an additional heating device for vehicles is disclosed. Here the gas cooler, or condenser, respectively, is divided into different regions, alternatingly used for cooling or heating. First, there is an evaporator region, which in cooling plant operation cools and, accordingly, dehumidifies the air flowing into the vehicle interior. Second, another region, in heat pump operation, heats the air flowing into the vehicle interior. This functional separation ensures that the air condensed on the evaporator will not, or only a little, be re-absorbed by the air flowing into the vehicle interior, thereby reducing possibility of flash fogging.
In DE 198 55 309, the heating heat exchanger is combined with the additional heating device from the refrigerant circuit for heating in such a way that the heat exchangers are switched in series. However, this results in the disadvantage that even more of the limited space available in the ventilation plants of motor vehicles is required by such a series connection.
Therefore, it is the objective of the invention to provide a heating heat exchanger, which requires little space and enables an advantageous control behavior and lowest possible flow resistance.